And let's listen to some more of the voices of the last 21 hours, as we've tried to make sense of what happened in Boston yesterday. One of those voices came earlier this morning. President Obama spoke at the White House.

PRESIDENT BARACK OBAMA: We know that two explosions gravely wounded dozens of Americans, and took the lives of others including a - 8-year-old boy.This was a heinous and cowardly act. And given what we now know about what took place, the FBI is investigating it as an act of terrorism.

DAVID GREENE, HOST:

Now, in that statement, the president also asked Americans to remain vigilant. He said the federal government is taking appropriate security measures.

Earlier this morning, law-enforcement officials gave an update on their investigation. They had very few leads to report, and said there were no suspects in custody. The public has been asked to submit any photos or videos from the scene. Here's another voice now, special agent in charge of Boston's FBI office, Richard DesLauriers.

RICHARD DESLAURIERS: Assistance from the public remains critical in establishing a timeline of events, which leads to swift conclusion through due diligence and strong investigative activity.

INSKEEP: Now, as investigators try to assemble that timeline, stories of heroismare emerging. As spectators fled to safety yesterday, emergency workers, runners and passers-by ran into the chaos, to help those who were wounded. Today, Boston's mayor, Tom Menino, spoke of the resilience of his city in the face of terror.

MAYOR TOM MENINO: This is a close-knit place, the city of Boston. Here, we know our neighbors; we grieve for them. We grieve for the little boy who we knew, from Dorchester. We know our heroes, also. They're the men and women who wear helmets, that wear the badges; the runners; who helped us yesterday during this time of need.

GREENE: And we'll be continuing to cover this story. We have reporters in Boston, at the White House and elsewhere. Stay with NPR News on the air, and online at npr.org, throughout the day as we learn more on this investigation and the victims of Monday's attack in Boston.